Citation and License

BMC Evolutionary Biology 2011, 11:21
doi:10.1186/1471-2148-11-21

Published: 20 January 2011

Abstract

Background

The notochord is a defining feature of the chordate clade, and invertebrate chordates,
such as tunicates, are uniquely suited for studies of this structure. Here we used
a well-characterized set of 50 notochord genes known to be targets of the notochord-specific
Brachyury transcription factor in one tunicate, Ciona intestinalis (Class Ascidiacea), to begin determining whether the same genetic toolkit is employed
to build the notochord in another tunicate, Oikopleura dioica (Class Larvacea). We identified Oikopleura orthologs of the Ciona notochord genes, as well as lineage-specific duplicates for which we determined the
phylogenetic relationships with related genes from other chordates, and we analyzed
their expression patterns in Oikopleura embryos.

Results

Of the 50 Ciona notochord genes that were used as a reference, only 26 had clearly identifiable orthologs
in Oikopleura. Two of these conserved genes appeared to have undergone Oikopleura- and/or tunicate-specific duplications, and one was present in three copies in Oikopleura, thus bringing the number of genes to test to 30. We were able to clone and test
28 of these genes. Thirteen of the 28 Oikopleura orthologs of Ciona notochord genes showed clear expression in all or in part of the Oikopleura notochord, seven were diffusely expressed throughout the tail, six were expressed
in tissues other than the notochord, while two probes did not provide a detectable
signal at any of the stages analyzed. One of the notochord genes identified, Oikopleura netrin, was found to be unevenly expressed in notochord cells, in a pattern reminiscent
of that previously observed for one of the Oikopleura Hox genes.

Conclusions

A surprisingly high number of Ciona notochord genes do not have apparent counterparts in Oikopleura, and only a fraction of the evolutionarily conserved genes show clear notochord expression.
This suggests that Ciona and Oikopleura, despite the morphological similarities of their notochords, have developed rather
divergent sets of notochord genes after their split from a common tunicate ancestor.
This study demonstrates that comparisons between divergent tunicates can lead to insights
into the basic complement of genes sufficient for notochord development, and elucidate
the constraints that control its composition.